FIG. 16 is a layout view showing the layout of a known protective relay for detecting an internal fault in an oil-filled transformer having a built-in on-load tap changer. Shown here are a transformer body 30, a radiator 31 mounted on the transformer body 30 for cooling oil, and an oil pump 32 mounted in the oil pipe running between the transformer body 30 and the radiator 31. Also shown are an oil purifier 33, a tap changer chamber 34, and conservators 35 and 36. Also shown are a Buchholtz's relay 37, a sudden pressure relay 38, and an oil flow relay 39. FIGS. 17 and 18 are diagrammatic cross-sectional views showing respectively the internal constructions of the Buchholtz's relay 37 and the sudden pressure relay 38 in FIG. 16. Since the oil flow relay 39 is identical, in construction and principle, to the Buchholtz's relay 37 its explanation is omitted here.
As shown in FIG. 17, a float 37a is provided within the Buchholtz's relay 37, and the float 37a floats or submerges with a floating support 37b depending on whether oil flow 201 is a rapid or slow. When the float 37a is submerged by a rapid oil flow, its associated contact points (contactors) 37c are closed, sending a signal externally via signal lines 37d.
As shown in FIG. 18, a partitioning plate 38a is provided in the middle of the sudden pressure relay 38, and the upper space above the. partitioning plate 38a forms a switch chamber 38b, in which a microswitch 38c is mounted. The partitioning plate 38a is provided with a hole in its center in which a bellows 38d is firmly engaged as shown, so that oil may not ingress into the switch chamber 38b. The partitioning plate 38a is provided with an equalizing capillary 38e which comunicates the switch chamber 38b with the interior of the bellows 38d to equalize both internal pressures. Disposed above the bellows 38d is a microswitch driving bellows 38f which expands upward by means of oil pressure 202 to activate the microswitch 38c when an oil pressure sudden or the like is generated.
The operation will now be discussed. When a fault associated with arcing takes place within the transformer body 30, the surrounding dielectric materials are quickly decomposed thermally and gasified, causing a rapid oil flow toward the conservator 36 and a sudden internal pressure rise. When the rapid oil flow takes place, the float 37a in the Buchholtz's relay 37 is forced to be submerged as shown in FIG. 17 by the dashed line, closing the associated contact points (contactors) 37c, whereby a signal is generated externally via the signal lines 37d. In the sudden pressure relay 38, the microswitch driving bellows 38f is expanded in response to the sudden oil pressure rise, whereby a contact closure signal for microswitch 38c is generated externally.
On the other hand, during normal operation, although a mild thermal expansion of oil takes place in the transformer body 30 as ambient temperature rises or a load increases, the resulting slow oil flow does not force the float 37a of the Buchholtz's relay 37 to submerge, and in the sudden pressure relay 38, the mild pressure change is equalized through the equalizing capillary 38e mounted between the bellows 38d and the switch chamber 38b, whereby the microswitch driving bellows 38f is not expanded upward, and a contact closure signal of the microswitch 38c is never generated.
Since the known protective relays in FIG. 16 are constructed as mentioned above, and the fault in the transformer body 30 is detected by the Buchholtz's relay 37 and the sudden pressure relay 38, the known protective relays suffer from the problem that when a sudden oil flow or a sudden pressure change takes place where the the oil pump 32 is started or stopped or in the event of an earthquake, the known protective relays arise malfunction by mistaking the change for a fault. There is also a considerable time lag from the moment an arc occurs to the moment of thermal decomposition of the dielectric and pressure rise. Furthermore, the mechanical relays mentioned above take time to operate, so a considerable time delay is involved until the transformer body 30 is stopped by the relay signals, whereby the fault area is widened in terms of mean time.
FIG. 19 shows the construction of a known differential relay used for detecting an internal shortcircuit fault in a transformer. In the figure, CT1 and CT2 denote current transformers respectively arranged on the primary side 40 and the secondary side 41 of the transformer, and the secondary windings of the current transformers CT1 and CT2 are connected to the operating coil of the differential relay 43 in a manner so that both transformers conduct current in mutually opposite directions. Current transformation ratios are determined based on the transformation ratio of the transformer so that the secondary currents of the current transformers CT1 and CT2 are approximately equal to each other.
The operation will now be discussed. When the transformer (40, 41) is operating normally, the secondary currents of the current transformers CT1 and CT2 are almost equal to each other, and a current (i1-i2) in the operating coil of the differential relay 43 is too small to activate the differential relay 43. However, when an accident such as shorted layer, grounding or the like takes place in the transformer 40, 41, the ratio of the primary current (I1) to the secondary current (I2) of the transformer becomes different from rated ratio, thereby increasing the current (i1-i2) in the operating coil, activating the differential relay 43, to thereby output the signal.
However the transformer 40, 41 is typically provided with a tap changer 44 and taps 45, thus the current (i1-i2) in the operating coil cannot always be zero when the transformer 40, 41 is operating normally. For this reason, some degree of dead zone should be allowed in the relay itself so that the differential relay 43 may not be activated by the current (i1-i2) in the operating coil during normal operation. Due to the dead zone, the known differential relays suffer from the problem that an initial minor layer short fault goes undetected, and the fault is detected later at its advanced stage in which a substantial number of windings are shorted.
FIG. 20 is a block diagram showing the construction of an internal fault sensor device of electrical equipment disclosed in Japanese Laid-open Patent Publication No. 63-247674. Shown in the figure are a switchgear 51 as an object to be monitored, and an electromagnetic wave signal 52, which is generated by an internal fault in the switchgear 51, and which has a diversity of frequency components determined by the circuit configuration and the length of line on which the switchgear 51 is placed. Designated 53A and 53B are a pair of loop antennas covering the medium frequency to very high frequency wave bands, which are spaced apart by 1 m or so with the planes of the loops mutually in parallel and at a right angle to a line which connects both loop antennas. The output of each of the loop antennas 53A and 53B is conducted to a junction connector via a coaxial cable. One loop antenna 53B is provided with a polarity changing switch 53b, so that the two loop antennas 53A and 53B can be set to the same or opposite polarities. Numeral 54 denotes a first amplifier which is either of a tuned type or non-tuned type. Also shown are a detector 55 connected to the first amplifier 54, a second amplifier 56 connected to the output side of the detector 55, a sound generator 57 connected to the output side of the second amplifier 56, and a pulse count display means 58 connected to the output side of the detector 55.
The operation will now be discussed. The electromagnetic wave signal 52 generated within the switchgear 51 is received by the antennas 53A and 53B, and fed via the coaxial cables and connectors to the first amplifier 54, where it is amplified into a first signal, and the detector 55 outputs a second signal which is the envelope of the first signal. The second signal contains extremely high frequency components and audio frequency components as well. The second signal is fed to the second amplifier 56 to be amplified there and then output as an audio sound by the sound generator 57, while the second signal is also fed to the pulse count display means 58 which counts and displays a frequency in excess of a set level. When the electromagnetic wave 52 originates in a line vertical to a line which connects the two loop antennas 53A and 53B, the output increases if both loop antennas are set to the same polarity, and the output decreases if both loop antennas are set to be opposite polarity, but as the electromagnetic source recedes from the vertical line, the difference between the same polarity and the opposite polarity is lowered. Taking advantage of this, the direction of radiation of the electromagnetic wave 52 can thus be found and the location of the fault generated detected.
The known internal fault sensor device in the electrical equipment in FIG. 20 is constructed as mentioned above accordingly, such electrical equipment suffers from problems such as noise from broadcasting waves, etc. is likely to interfere since broadcasting systems widely use frequency bands from medium wave to very high frequency wave, the operation is complex since the position and alignment of the pair of loop antennas 53A and 53B need to be changed while manipulating the polarity changing switch 53b to find the direction, the difficulty of minituarizing, handling and mounting the device since the separation dimension between the antennas 53A and 53B, is as large as 1 meter.
FIG. 21 is a schematic diagram of a known fault monitoring device used in electrical equipment as disclosed by Japanese Laid-open Patent Publication No. 7-128393. Shown in the figure are a high-voltage bus 61 as an object to be monitored, a voltage transformer 62 connected to the high-voltage bus 61, a switching element 63 connected to the secondary side of the voltage transformer 62 that responds to faults other than dielectric faults, a discharger 64 connected to the secondary side of the voltage transformer 62, in parallel with the switching element 63, an electromagnetic wave 65 generated by the discharge of the discharger 64, an antenna 66 for receiving the electromagnetic wave of a partial discharge, and a fault sensor device 67 connected to the antenna 66.
The operation will now be discussed. When a fault other than a dielectric fault takes place, the switching element 63 is opened, and the voltage into which the voltage transformer 62 divides the voltage of the high-voltage bus 61 is applied to the discharger 64, whereby the discharger 64 starts discharging. The electromagnetic wave 65 is radiated by the discharging and is received by the antenna 66. The received signal at the antenna 66 is sent to the fault sensor device 67, by which the generation of the fault is detected.
Since this known fault monitoring device for use in electrical equipment is constructed as shown in FIG. 21, the electromagnetic wave 65 must be artificially generated by the switching element 63, the voltage transformer 62 and the discharger 64 to detect faults other than partial discharges. Since the device responds to partial discharges, it is difficult to discriminate its fault from partial discharges even though they are completely different phenomenas.
FIG. 22 is a block diagram showing the construction of a known fault sensor device used in a gas insulated switchgear as disclosed by Japanese Laid-open Patent Publication No. 3-139110. Shown in the figure are a gas container 70 of the switchgear, a plurality of main circuit conductors 71 disposed in the gas container 70, and an insulating spacer 72 which passes through and supports the main circuit conductors. Numeral 73 denotes an electric-field moderating electrode embedded in the insulating spacer 72, which is connected to a mounting nut and bolt 75 via a connecting bolt 74 used as an antenna. The mounting nut and bolt 75 is insulated from the gas container 70 by an insulating washer (not shown), and is connected to the ground via a low-pass filter 76. Numeral 77 denotes a receiver for picking up electromagnetic waves of a particular frequency such as 100 MHz or so, for example, and the mounting nut and bolt 75 is connected to the input side of the receiver 77 via a coaxial cable 78. Numeral 79 denotes a signal process determining module connected to the output side of the receiver 77.
The operation will now be discussed. When a partial discharge occurs in the gas container 70, the electromagnetic wave (not shown) generated by the partial discharge is received by the electric-field moderating electrode 73 in the insulating spacer 72, and input to the receiver 77 via the connecting bolt 74 and the mounting nut and bolt 75, and is detected and judged by the signal process determining module 79.
Since the known fault sensor device for the gas insulated switchgear in FIG. 22 is thus constructed, the distance between the main circuit conductors 71 and the electric-field moderating electrode 73 is short compared with the wavelength of the electromagnetic wave to be received (the wavelength is 3 m in the gas and 1.7 m in the spacer material), the fault sensor device responds to a frequency component of the oscillation voltage of the main circuit conductors 71 that happens to be the same frequency as the response frequency of the receiver, rather than to the electromagnetic wave (radiation electric field), and furthermore, since even travelling high-frequency oscillation voltages occuring outside of the switchgear, such as coronas generated on the bus in the air connected to the switchgear are detected, the possibility of erroneous sensing a rises and noise levels cannot be reduced. Since the detection is chiefly dependent on electrostatic coupling with the main circuit conductors 71, a discharge in gas-filled space remote from the main circuit conductors 71 is difficult to detect.
As mentioned above, in the protective relays in FIG. 16, the fault sensor device suffers from the problems that the fault sensor device is activated by an internal oil flow or a change in oil pressure which is not caused by an internal fault in the electrical equipment, and that the fault area is widened since a considerable time is needed until the oil flow and pressure change increase enough and the mechanical operation of the fault sensor device itself is completed. The known differential relay used in the transformer shown in FIG. 19 suffers from the problem that a fault is detected only when the fault area is substantially widened since a dead zone is needed in consideration of the tap change. The known internal fault sensor device shown in FIG. 20 suffers from the problem that a pair of loop antennas are needed for direction finding, thus its operation is complicated involving the polarity changing step, and also its large dimensions present difficulties in handling the device. In the known fault monitoring device for use in electrical equipment in FIG. 21, a special mechanism is needed to artificially generate an electromagnetic wave in response to a fault other than dielectric faults, thus presenting the difficulty of discriminating between a partial discharge and other faults which are the different type of phenomena from the discharge. In the known fault sensor device in gas insulated switchgear shown in FIG. 22, the distance between the antenna and the main circuit conductors to which high-frequency oscillation voltages travelling from outside of the switchgear due to a diversity of causes are fed is short compared with the wavelength, the antenna is placed in the near field where the effect of electrostatic field is dominant, and thus the fault sensor device suffers from the problem that the detection of the fault is influenced by noise propagating from sources other than the subject electrical equipment, and that the level of detection for discharges or the like in the gas-filled space remote from the main circuit conductors is lowered.